Set of blades for a turbine

ABSTRACT

A set of blades for a turbine which includes a group of blades disposed between an upper plate (12) and a lower plate (11) which delimit a group of passages (3). Said set of blades includes ducts (13,14) provided in the upper plate (12) and/or in the lower plate (11), each duct opening at one end in the neighborhood of the convex surface of a blade and level with the constriction (6) of the passage and at the other end at a point situated upstream from said set of blades. Reduction of the low pressure on the convex surfaces only adjacent the upper plate (12) and the lower plate (11), reducing secondary losses, while pressure distribution remains optimum in the zone of good flow of the passage.

FIELD OF THE INVENTION

The present invention relates to a set of blades for a turbine whichincludes a group of blades disposed between an upper plate and a lowerplate. The group of blades therefore defines a group of passages alongwhich a fluid flows, each passage being delimited by the convex surfaceof one blade and the concave surface of the blade next to it and by thelower plate and the upper plate.

BACKGROUND OF THE INVENTION

It is known that in a given passage, at points which are sufficientlyfar from the walls of the passage, the stream lines follow paths whichare substantially parallel to the walls of the passage formed by theconcave and convex surfaces of the blades. At all points along the path,the centrifugal force which is exerted on a particle is balanced by thepressure forces. The result of this is, generally, that the concavesurface of the blade is subjected to a higher pressure than is theconvex surface of the other blade which delimits the passage.

It is also known that in the boundary layer near the lower plate andupper plate, the speed of the fluid is low. It follows that the pressureforces are no longer balanced and the stream lines are curvesperpendicular to the isobars and follow paths of considerable slippagein each passage from the concave surface to the convex surface as iswell known to the person skilled in the art (see, for example, thearticle in the November 1941 French issue of the Brown Boveri review--p.356 to 361 and, in particular FIGS. 2 and 3).

The slippage generates a counter-clockwise eddy against the upper plateof the passage and a clock-wise eddy against the lower plate as seen byan observer placed downstream from the set of blades.

These disturbances cause important losses known as secondary losses andthe smaller the ratio between the height of the blades and the chord,the more the efficiency of a set of blades is reduced.

A known means for reducing the secondary losses consists in reducing theaerodynamic loading on the blades. This is equivalent to reducing thedifference in average pressure between their concave and the convexsurfaces e.g. by reducing the blade spacing to chord ratio of the set ofblades.

However, the disadvantage of the above method is that it increases thefriction losses in the zone of good flow along the passage in such a waythat the gain obtained on the secondary losses can be cancelled byincreasing the friction losses in the main flow.

SUMMARY OF THE INVENTION

The set of blades in accordance with the present invention which allowsthe secondary losses to be reduced without having any influence on thefriction losses in the main flow is characterized in that for eachblade, at least one duct is provided in the upper plate, said ductopening at one end in the neighbourhood of the convex surface of saidblade and level with the constriction of the passage which is delimitedby the convex surface of said blade and at the other end at a pointsituated upstream from said set of blades and/or at least one duct isprovided in the lower plate and opens at one end in the neighbourhood ofthe convex surface of the blade and level with the constriction of thepassage which is delimited by the convex surface of said blade and atthe other end at a point situated upstream from said set of blades. Theconstriction of the passage is the place where the convex surface of oneblade and the concave surface of the blade next to it are the nearesttogether, said blades delimiting said passage.

The point situated in the neighbourhood of the convex surface of theblade in the constriction of the passage is the point of the passagewhere the pressure is at its minimum. Due to the set of blades inaccordance with the invention, the low pressure on the convex surface isreduced by a duct which leads to a higher-pressure chamber. This reducesthe difference in average pressure between the convex surface and theconcave surface. However, the reduction in pressure takes place in onlya zone adjacent the lower plate and/or the upper plate of the passage,thus reducing the secondary losses, while the aerodynamic loadingremains optimum in the portion of good flow along the passage. Thereduction of the low pressure adjacent the convex surface reduces theslippage of the boundary layers and the losses due to secondary flow.

In the case where the set of blades does not include an upper plate(some moving blades) or a lower plate (some fixed blades), the passagesare then formed only in the lower plate (moving blades) or in the upperplate (fixed blades).

The following description given with reference to the accompanyingdrawings will make it easier to understand how the invention can beimplemented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view which illustrates part of the set of bladesof a conventional turbine;

FIG. 2 is an axial cross-section of a turbine which includes sets ofblades in accordance with the invention.

FIG. 3 is a transversal cross-section seen from above of a set of bladesin accordance with the invention.

DESCRIPTION OF THE PRIOR ART AND THE PREFERRED EMBODIMENT

FIG. 1 illustrates two blades A and B which form a part of a prior artturbine and whose roots are fixed to a lower plate 1 while their headsare fixed to an upper plate 2. The lower plate and the upper plate areusually cylindrical or frustoconical surfaces.

A passage 3 is delimited by the concave surface of the blade B, theconvex surface of the blade A, the lower plate, and the upper plate. Thepassage 3 has a constriction 6 (see FIG. 3) which is the point where theconvex surface of the blade A is nearest the concave surface of theblade B.

Fluid far from the walls of the passage flows smoothly along streamlinessuch as (c). In contrast, streamlines of fluid which come into contactwith the upper plate and the lower plate are orthogonal to the isobarsand flow in the directions shown (l) and (m), then begin to be turbulentas soon as they strike the convex surface of the blade (A).

FIG. 2 illustrates an axial cross-section of a turbine fitted with setsof fixed blades 4 and moving blades 5 in accordance with the invention.

For each blade 4 of the set of fixed blades, a duct 13 is provided inthe lower plate 11 between a point in the neighbourhood of the concavesurface of the blade and opening to the constriction 6 of the passage,said point being the point where the pressure along the convex surace isat its minimum, and a point upstream from the set 4 of blades (see FIG.3) in a zone where the pressure is high.

Also for each blade 4 of the set of fixed blades, a duct 14 is providedin the upper plate 12 between a point in the neighbourhood of the convexsurface of the blade and opening to the constriction 6 of the passage,and a point upstream from said set of blades in a zone where thepressure is high.

For each blade 5 of the set of moving blades, a duct 24 is provided inthe upper plate 22. Said duct begins at a point in the neighbourhood ofthe convex surface of the blade and opening to the constriction 6 of thepassage (minimum pressure point) and ends upstream from said set ofblades 5 (high-pressure zone).

Also, for each blade 5 of the set of moving blades, a duct 23 isprovided in the lower plate 21. Said duct begins at a point in theneighbourhood of the convex surface of the blade and opens to theconstriction 6 of the passage (minimum pressure point) and ends upstreamfrom the set of blades 5 (high-pressure zone).

The passages between a point of the lower plate and/or of the upperplate at the minimum pressure points of the convex surfaces of theblades and a point in a high-pressure chamber constituted by the portionsituated upstream from the blades make it possible to reduce the lowpressure in the neighbourhood of the convex surfaces of the blades andlevel with the lower plate and/or with the upper plate.

It is self-evident that the turbine in accordance the the inventioncould be provided with such ducts only on the sets of moving blades oron the sets of fixed blades in the lower plate and/or upper plate ofthese sets of blades.

Further, these ducts could be associated with only some blades of thesame set.

We claim:
 1. In a set of blades for a turbine, said set of bladesincluding a group of blades bearing opposite convex and concave surfacesdisposed between an upper plate and a lower plate which delimite a groupof passages, the improvement wherein, for each blade, said set includesat least two ducts (13, 14), one in the upper plate (12) and the otherin the lower plate (11), each duct (13, 14) opening at one end in theneighbourhood of the convex surface of the blade and at the constriction(6) of the passage which is delimited by the convex surface of saidblade and, in the other end, at a point situated upstream from said setof blades.
 2. In a set of blades for a turbine, said set of bladesincluding a group of blades having opposite convex and concave surfacesdisposed between an upper plate and a lower plate which delimit a groupof passages, the improvement wherein, said set includes ducts in theupper plate (12) which are associated with the blades (14), each ductopening at one end in the neighbourhood of the concave surface of theblade at the constriction (6) of the passage which is delimited by theconvex surface of said blade and, at the other end, at a point situatedupstream from said set of blades.
 3. In a set of blades for a turbine,said set of blades including a group of blades having opposite concaveand convex surfaces disposed between an upper plate and a lower platewhich delimit a group of passages, the improvement wherein, said setincludes ducts in the lower plate (11) which are associated with theblades (13), each duct opening at one end in the neighbourhood of theconvex surface of the blade at the constriction (6) of the passage whichis delimited by the convex surface of said blade and, at the other end,at a point situated upstream from said set of blades.
 4. In a set ofmoving blades for a turbine, said set of moving blades including a groupof blades having opposite concave and convex surfaces assembled on alower plate which delimit a group of passages, the improvement wherein,said set includes ducts in the lower plate (21) which are associatedwith the blades, each duct opening at one end in the neighbourhood ofthe convex surface of the blade at the constriction of the passage whichis delimited by the convex surface of said blade and, from the otherend, at a point situated upstream from said set of blades.
 5. In a setof fixed blades for a turbine, said fixed blades including a group ofblades having opposite concave and convex surfaces assembled on an upperplate which delimit a group of passages, the improvement wherein, saidset includes ducts in said upper plate (12) which are associated withthe blades, each duct opening at one end in the neighbourhood of theconvex surface of the blade at the constriction of the passage which isdelimited by the convex surface of said blade and, at the other end, ata point situated upstream from said set of blades.